1. Field of the Invention
The present invention relates to an image encoding/decoding apparatus for high efficiency encoding/decoding of image information.
2. Related Background Art
In recent years, in the field of digital transmission of a moving image, research of a high efficient encoding technique has been positively performed and image encoding/decoding apparatuses using various encoding methods have been proposed. As one of those methods, there is an encoding method whereby image information is divided into a plurality of frequency bands and optimum encoding is performed on each of the divided signals. FIGS. 1A and 1B are block diagrams of the conventional image encoding/decoding apparatus using this method.
An analog image signal supplied to an input terminal 110 is converted to a digital signal by an analog/digital (A/D) converter 112. A frequency band dividing circuit 114 comprising a digital filter or the like divides the digital image signal which is output from the A/D converter 112 into a plurality of signal groups of predetermined frequency bands. In the example of FIGS. 1A and 1B, the frequency band of the original signal is divided into four equal bands and are set to LL, LH, HL, and HH from the lower frequency side. The frequency band dividing circuit 114 supplies the signal (or signal group) LL on the lowest frequency side, the signal (or signal group) LH, the signal (or signal group) HL, and the signal (or signal group) HH on the highest frequency side to high efficiency encoding circuits 116, 118, 120, and 122, respectively.
As for the division of the frequency band, if the number of data after completion of the division to four frequency bands is equal to the original number of sample data, the number of data is increased to the value that is four times as large as the number of original sample data. Therefore, from a viewpoint of information compression, a process for down-sampling each signal to 1/4 size is often executed. In order to prevent a loss of information at the time of filtering, there are many cases where filter characteristics for the frequency band division are set so as to be partially overlapped with each other.
In general, when a signal is divided into a few frequency bands as mentioned above, electric power is often concentrated to a special frequency band, ordinarily, to the low frequency side. The signals LL, LH, HL, and HH produced by the frequency band dividing circuit 114 have different characteristics. By using those deviations, the high efficiency encoding circuits 116 to 122 encode the signals LL, LH, HL, and HH by encoding methods according to the signal characteristics.
The signals encoded by the high efficiency encoding circuits 116 to 122 are multiplexed by a multiplexing circuit 124 and the multiplexed signal is supplied to an error correction encoding circuit 126. The error correction encoding circuit 126 adds (error correction encoding) a parity bit for code error correction to the multiplexed data from the multiplexing circuit 124. A modulation circuit 128 modulates the multiplexed data which was error correction encoded by the error correction encoding circuit 126 in accordance with the characteristics of a transmission line 130, for example, by a QPSK method in case of satellite transmission and transmits the modulated signal to the transmission line 130.
On the reception side, a demodulation circuit 132 demodulates a data train which is input from the transmission line 130. An error correction decoding circuit 134 corrects the transmission error by using the parity bit. In the case where an uncorrectable error occurs, the error correction decoding circuit 134 sets an error flag to the high level so as to enable the encoding data including the error to be corrected by replacing it with other data upon decoding. Such a process is well known.
The error corrected multiplexed data is input to a division circuit 136. The division circuit 136 divides the input data into four frequency bands in a manner similar to the dividing process at the time of encoding. In this case, delimiter information of each data has previously been inserted at the time of data multiplexing and it is now assumed that the data division is executed on the basis of the delimiter information.
The four data divided by the division circuit 136 are sequentially supplied to high efficient decoding circuits 138, 140, 142, and 144 in accordance with the order from the low frequency side. The decoding circuits 138 to 144 perform the decoding processes corresponding to the encoding circuits 116 to 122 on the transmission side. Namely, the decoding circuit 138 generates a decoded signal of the signal LL in the lowest frequency band, the decoding circuit 140 generates a decoded signal of the signal LH, the decoding circuit 142 generates a decoded signal of the signal HL, and the decoding circuit 144 generates a decoded signal of the signal HH.
The signals of the frequency bands decoded by the decoding circuits 138 to 144 are supplied to a frequency band synthesizing circuit 146 and is frequency band synthesized by a process opposite to the process at the time of frequency band division. The frequency band synthesizing circuit 146 comprises a digital filter or the like in a manner similar to the frequency band dividing circuit 114. In the case where the down-sampling process has been executed at the time of frequency band division, an up-sampling is also executed as an opposite process before filtering.
In the case where an uncorrectable error occurred in the error correction decoding circuit 134 although not shown, the uncorrectable image data is corrected at the front stage or post stage of the frequency band synthesizing circuit 146 on the basis of the flag information.
The digital image signal reconstructed as mentioned above is converted into an analog signal by a D/A converter 148 and is output as an analog image signal from an output terminal 150 to the outside.
However, in the conventional apparatus, in the case where an uncorrectable error occurs on the transmission path, the image correction is performed as mentioned above, so that image quality deteriorates. Particularly, even in case of the same errors, when an uncorrectable error occurs in the encoded data in a low frequency band which is most important upon decoding of the image signal, there is a situation such that a degree of deterioration of the image quality increases even if the image is corrected. Its influence is larger than that by a degree of deterioration of the image quality in the case where an uncorrectable error occurs in the encoded data in high frequency regions other than the low frequency region.
In the conventional apparatus, similar processes are performed on errors which occur on the transmission path with respect to any one of the divided frequency bands. In other words, nothing is considered about a countermeasure to especially prevent a large deterioration of the image quality due to an error of the data in the low frequency band.